1. Field of the Invention
The invention relates to a process for making polyvinylidene fluoride (xe2x80x9cPVDFxe2x80x9d) powder coatings using a precipitation process in aqueous media. Specifically, the process involves the use of coagulation to precipitate PVDF and an acrylic resin blend from an aqueous dispersion prepared from blending PVDF latex and a water reducible acrylic resin solution or dispersion. The invention also involves pigmented PVDF-based powder coating compositions made by the coagulation process.
2. The Prior Art
PVDF coating compositions can be used for coating a large variety of metal substrates, including aluminum, steel and galvanized steel, and has wide acceptance as an architectural coating due to the properties of PVDF, including high crystallinity and photo-oxidative resistance, which provide for coatings that are weather resistant and stand up to corrosive or other harsh environments. In order to balance the performance of PVDF, a secondary resin is normally needed to, among other things, provide good adhesion to substrate and to reduce the shrinkage of the polyvinylidene fluoride due to excess crystallization. The secondary resin, which may be a thermoplastic polymer, enhances the film forming capabilities of the PVDF-based coating.
Commercial PVDF coating materials are generally solvent dispersions and release solvent during coating preparation. Solvent emission is generally managed by the end user through the installation and operation of an incineration system in a coating line. The incinerator will burn the VOC of the solvent emission to reduce or preclude harmful emission of VOC to the atmosphere. The need for an incinerator is, generally, a regulatory requirement. Thus, PVDF based paints in a solvent dispersion will require the end user to invest capital to install air pollution control apparatus, such as an incinerator, and incur costs to maintain the equipment and comply with regulatory requirements. These costs can be avoided by the use of solvent free coatings, such as PVDF-based powder coating preparations.
Additional benefits are obtained through use of solvent-free PVDF coating materials, such as powder coatings. Powders can be sold and shipped in a ready to use state thereby obviating any need by the end user for mixing, stirring or thinning during application. Powder based coatings can be used more efficiently than wet spray paint and the powder coatings have a higher percentage usage because the powder can be recycled during coating operations. The high raw material utilization associated with powder coating arises from the lack of any solvent requirement for application. Also, because the powder has no VOC there is significantly reduced absorption of energy during film forming on a substrate after application thereby resulting in energy savings by the end user during application. These, and other, advantages of PVDF-based powder coating compositions result in consumer and industrial demand for such coating products.
Because of the benefits of powder based coatings, there is increasing industrial and consumer demand for such coating compositions. Accordingly, the art is constantly evolving with novel processes for making PVDF-based powder coatings. The art is also constantly seeking novel PVDF-based coating materials with improved properties over existing coating compositions and formulations.
For example, relatively recent processes are described in U.S. Pat. Nos. 4,770,939 and 5,346,727 for obtaining PVDF-based powder coatings by cryogenic grinding of a melt extruded combination of PVDF and compatible thermoplastic. U.S. Pat. No. 5,229,460 describes a grinding process for making PVDF-based powder coatings that does not involve cryogenic conditions. A method for making a pigmented PVDF powder coating using a solvent removal process of a solvent based dispersion paint, but without coagulation of a polymer latex mixture, is described in U.S. Pat. No. 5,739,202. A process for making a powder comprising a (meth)acrylate polymer and fluoropolymer by combining two polymer latex phases without coagulation is described in U.S. Pat. No. 5,827,608. Some of the processes in the art are inefficient, and other processes in the art do not result in a PVDF-based powder coating that has acceptable pigmentation.
It was an object of the invention to develop an efficient coagulation method for making pigmented PVDF-based powder coating compositions.
It was a further object of the invention to develop a coagulation process that results in a powder coating that has sufficient pigmentation to provide even coloring when heat cured after application to a substrate.
It was another object of the invention to develop a coagulation process for making PVDF-based powder coatings that reduces the need for additives, such as surfactant, yet provide for an adequately pigmented coating.
It was another object of the invention to develop PVDF-based powder coating compositions having appropriate pigmentation and suitable properties for industrial coatings.
These, and other objects of the invention, are achieved by the process described herein involving coagulation of PVDF latex and a water reducible acrylic resin solution or dispersion. The aqueous acrylic resin comprises polymer chains containing side chain ionic moieties, which anchor onto the surface of pigment to form a stable acrylic polymer/pigment dispersion which does not require a surfactant to form. The acrylic polymer/pigment dispersion and PVDF latex are precipitated to obtain the pigmented powder coating composition. The pigmented powder coating composition can be applied to any number of substrates and is heat cured forming a pigmented film over the substrate having even and full color.
In the present Specification, all parts and percentages are by weight/weight unless otherwise specified.
The invention concerns coagulation of PVDF latex and a water reducible acrylic polymer solution or dispersion, containing either cationic or anionic species. The water reducible acrylic polymer phase further comprises pigment. The coagulation is induced by the addition of acid or base coagulant. For anionic species, acid coagulant is used and for cationic species, base coagulant is used. This coagulation process precipitates a desired homogeneous solid blend. After solid separation and spray drying, a powder suitable for powder coating is produced.
The PVDF-based powder coating compositions are generally applied to a substrate in powder form. The powder coated substrate is heated to fuse the PVDF and the water reducible polymer, preferably in the form of an acrylic polymer/pigment dispersion, in a continuous film coating on the substrate. In order to obtain the continuous film coating, the PVDF and water reducible acrylic polymer must be miscible during and after fusion, and the water reducible acrylic polymer is selected for the powder coating composition to have this miscibility.